Since MediaWiki 1.25 I see that during editing a page with the normal wiki editor, it makes a POST request to api.php, passing a action=stashedit
Since it's a POST request, it's somewhat difficult to see what's the specific api action it triggers. Also, looking at the RELEASE NOTES for 1.25 I don't see anything obvious about it, except this:
> * Edits are now prepared via AJAX as users type edit summaries. This behavior
> can be disabled via $wgAjaxEditStash.
Well, that's not even accurate, since the POST request is made when focusing the textarea, not the edit summary. What does "Edits are now prepared" actually mean? If I focus the text area, what's the purpose of sending the textarea contents if I haven't even started to modify it? What's the benefit of "preparing an edit" that outcomes the additional server load?
[[ https://www.mediawiki.org/wiki/Manual:$wgAjaxEditStash | Manual:$wgAjaxEditStash ]] is more of the same, it doesn't explain anything useful, apart that saying you can disable it. No documentation about benefits, what won't work if you disable this feature, etc